/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Register another window that may be visible in a non-top level way that still needs to be able to maintain focus paths. Generally speaking - this is for Virtual Windows that are created to render in the 3D world slate content.
void RegisterVirtualWindow ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > InWindow )